1. Field of the Invention
The present invention relates to an image recording apparatus corresponding to high-speed scanning with a multibeam, and more particularly, it relates to an image recording apparatus employing a spiral multibeam scanning system.
2. Background of the Invention
In general, multibeam exposure is widely carried out in correspondence to increase of an image recording speed. FIG. 33(a) shows a scanning method which is generally employed in such multibeam exposure. Referring to FIG. 33(a), an exposure head 40 comprising a plurality of (n) light emitting elements moves in a subscanning direction X along a central axis 37 of a cylinder 36. On the other hand, the cylinder 36 rotates in a main scanning direction -Y in a prescribed rotation cycle. Consequently, a plurality of light beams which are emitted form the exposure head 40 arc imaged on a film 35 through a lens 41, whereby exposure pixels 42 are scanned and formed n the main scanning direction Y following the rotation of the cylinder 36. When scanning with the light beams following single rotation of the cylinder 36 in the main scanning direction -Y is completed, the exposure head 40 moves in the subscanning direction X, to again carry out scanning with the light beams in the main scanning direction Y. This scanning is repeated m times, so that the film 35 is scanned with n by m light beams, as shown in FIG. 34(a). Referring to FIG. 34(a), it is assumed that the exposure head 40 shown in FIG. 33(a) has n light emitting elements. It is also assumed that the number of scanning line areas SL (corresponding to areas exposed by the exposure head 40 in single main scanning) is m. Referring to FIG. 34(a), therefore, first, second, third, . . . , (m-1)-th and m-th scanning line areas SL from the left side are denoted by SL1, SL2, SL3, . . . , SL(m-1) and SLm respectively. The scanning line areas are hereinafter denoted in this manner.
In the method shown in FIG. 34(a), however, the cylinder 36 (see FIG. 33(a)) rotates by a turn while the exposure head 40 moves in the subscanning direction X, and no scanning is carried out in the main scanning direction Y during this rotation. Thus, the cylinder 36 rotates in vain. Consequently, implementation of a higher speed is hindered by such excessive rotation of the cylinder 36.
In order to solve this problem, there has been proposed a method of spirally scanning the film with light beams by continuously feeding the exposure head. As shown in FIG. 34(b), an image forming region 34a on the film is in the form of a parallelogram in this case and scanning line areas SL are inclined at an angle .theta. with respect to the main scanning direction Y, and hence an image as formed is distorted. This inclination angle .theta. is increased as the number n of beams is increased. In order to prevent such image distortion, therefore, it is necessary to bring the respective scanning line areas SL into rectangular shapes as shown in FIG. 35(a). Thus, it is necessary to incline arrangement of exposure pixels which are imaged on the film at the aforementioned angle .theta. with respect to the central axis 37 (see FIG. 33(a)) of the cylinder 36 in continuous feeding of the exposure head in the subscanning direction X.
Such a technique is disclosed in Japanese Patent Laying-Open Gazette No. 58-111566 (1983). In this technique, the exposure head 43 having n light emitting elements itself is inclined at the angle .theta. with respect to the central axis 37 (see FIG. 33(b)). Referring to FIG. 33(b), numeral 45 denotes exposure pixels which are imaged on the film 35, and numeral 44 denotes a lens. In this case, scanning line areas SL which are formed on the film 35 are displaced from those adjacent thereto, as shown in FIG. 35(a). As disclosed in the aforementioned gazette, it is possible to correct such displacement by delaying exposure timing in the main scanning direction Y every scanning line area SL. As the result of such correction, it is possible to form an image having neither image distortion nor displacement, as shown in FIG. 35(b).
While the aforementioned prior art discloses a principle which is related to spiral multibeam scanning, the same cannot properly cope with change in resolution of exposure pixels. While a proper magnification is set every image recording, the resolution of exposure pixels which are formed on the film is also inevitably changed upon change of the magnification. Consequently, the width .DELTA.X0 (see FIG. 35(a) or 35(b)) of each scanning line area is also changed and the inclination angle .theta. is influenced by this. Following such change of the inclination angle .theta., it is necessary to also take influence on both the moving speed of the exposure head in the subscanning direction and exposure timing in the main scanning direction into consideration. The aforementioned prior art has no disclosure about these points.
The aforementioned prior art is on the premise that the cylinder uniformly rotates in the main scanning direction and the exposure head is uniformly translated in the subscanning direction in a state inclined at a prescribed angle with respect to the subscanning direction. In practice, however, driving signals for controlling the rotation of the cylinder and the movement of the exposure head are not regularly maintained at ideal constant levels but slightly fluctuate within certain ranges about set values. Further, these driving signals are also influenced by change in the ambient temperature to fluctuate. Therefore, the rotational speed (hence, the rotation cycle) of the cylinder and the moving speed of the exposure head in the subscanning direction fluctuate during scanning with the light beams. The aforementioned prior art scans the film with a plurality of light beams along the main scanning direction while simultaneously scanning the same with these light beams also along the subscanning direction. Due to the aforementioned change of the speed, therefore, pitches between adjacent scanning lines in boundary portions between adjacent pairs of scanning line areas differ from those of the scanning lines belonging to the scanning line areas. Such difference leads to stripe-shaped image irregularity in the boundary portions. Even if the aforementioned change of the speed itself is small, such image irregularity substantially regularly takes place every boundary, and hence the image irregularity is rendered extremely conspicuous to deteriorate the quality of the recorded image.
To this end, awaited is spiral multibeam scanning previously taking fluctuation of the rotation speed of the cylinder and that of the moving speed of the exposure head into consideration, while it is necessary to make the respective parts in the apparatus correctly controllable in response to the value of resolution of exposure pixels (the number of exposure pixels or scanning lines per unit length in the direction of arrangement of the exposure pixels). The aforementioned prior art has no disclosure and no teaching in this point either.